Memories of Voyager

Voyager at Jupiter, Credit: NASA/JPL

The recent Alpbach summer school featured a fascinating presentation from a retired JPL employee, John Casani, who had played a major role in past exploration of the outer solar system.  As his stories included tales I hadn't heard before, about topics that have greatly affected my own research, I've decided to put some of them to paper.  The twin Voyager spacecraft started life in 1961 when Michael A. Minovitch discovered that the gravity of the planets could be used to slingshot spacecraft between different planetary targets.  Four years later, Gary A. Flandro discovered a unique planetary alignment that would recur once every 175 years, which would be ideal fur the use of gravity assists to reach the frigid planets of the outer solar system.  Such an approach would allow robotic explorers to reach Neptune much faster than a direct trajectory.  

The Grand Tour mission was proposed in 1970 for a "new start" in 1971, and originally consisted of two missions: one to the gas giants and Uranus, and one to the gas giants and Neptune.  However, NASA canned this concept due to concerns about its ambitious and costly nature, and JPL re-proposed a more modest mission to Jupiter and Saturn based on the Mariner series of spacecraft (Mariner Jupiter Saturn 1977).  That mission became Voyager in 1978, and the idea that the spacecraft could go on to visit the ice giants after the successful completion of the gas giant studies was introduced.  Casani described Uranus and Neptune as "targets of opportunity", rather than destinations which were required for NASA to consider Voyager a success, and explains why planetary mission goals often seem modest (e.g., mars rovers for 90 days) but go onto achieve great things.  It also meant extended funding would be contingent on the success of the gas giant mission.

The Voyagers were launched by Titan IIIE‐Centaurs on August 20, 1977 (V2) and September 5, 1977 (V1) at a then‐year cost of $320M. The spacecraft carried six RTGs for power, half of the paid for under the “Atoms for Peace” policy.  Major redesigns were required after Pioneer 11 discovered Jupiter's intense radiation belts in 1974, requiring radiation-hard components, a key feature of all Jupiter missions since.  Casani recalled that fumes from facility painting 3 weeks before launch meant that several science instruments required replacement detectors. 

The Voyagers went on to deliver a spectacular mission of discovery in the outer solar system, passing Jupiter in 1979, Saturn and Titan in 1981, and remain the only spa craft to ever visit the ice giants Uranus and Neptune in 1986 and 1989, respectively.  Both spacecraft are now leaving our solar system, with Voyager 1 the most distant human-made object at over 120AU from the sun in February 2012, and adding 3.6AU to that tally every year.  Power from those RTGs is declining, but four of the 11 science instruments are still operating.  By 2020, there'll likely only be enough for one instrument, recording the passage through the heliopause and into interstellar space until around 2025.

Strange Weather at the Society for Popular Astronomy

At a meeting of the Society for Popular Astronomy in London on 27 October 2012, I gave a presentation explaining how recent spacecraft observations have transformed our understanding of the atmospheres of the giant planets -- Jupiter, Saturn. Uranus and Neptune. Why do the belts of Jupiter sometimes disappear? What causes the storms on Saturn? Even amateur observations are helping to provide some answers..... watch the video below (Vimeo, 74 minutes).


Dr Leigh Fletcher: Strange Weather! Exploring the Giant Planets from Society for Popular Astronomy on Vimeo.

Saturn's Stratopheric Vortex

VLT image of Saturn's giant vortex at mid-infrared wavelengths, 13.1 µm, in July 2011.  The vortex formed from the merging of two pockets of warm air in the stratosphere. The two warm air masses, in turn, are an aftereffect of the 'Great Springtime Storm', a turbulent storm that affected Saturn's lower atmosphere from December 2010 until mid-2011.  At its biggest, in late June 2011, the vortex covered about 62 000 km - almost one quarter of the planet's circumference at the mid-northern latitudes affected by the storm.   Image courtesy of L.N. Fletcher, University of Oxford, UK, and ESO

After 18 months of continuous work on this project, I’m happy to say that our paper tracking the evolution of Saturn’s enormous stratospheric vortex (at its formation, the largest vortex in the solar system) is now out in the journal Icarus.  The vortex is an after-effect of the springtime storm on Saturn that I wrote about here, and is still present today.  This ‘beacon of infrared emission’, so called because it dominates the infrared light from the planet, is moving around the springtime hemisphere as regular as clockwork.  The Icarus paper can be found here:

L. N. Fletcher, et al., "The origin and evolution of Saturn's 2011-2012 stratospheric vortex", 2012, Icarus, Volume 221, Issue 2, November-December 2012, Pages 560-586,

http://dx.doi.org/10.1016/j.icarus.2012.08.024

Three press-releases are available from ESA and NASA.  The most indepth, by Claudia Mignone on ESA’s SciTech website, is included below, and features some great animations from Chrisophe Carreau. 

Saturn's giant storm reveals the planet's churning atmosphere

Emily Baldwin has written an overview of the discovery for ESA:

After-effects of Saturn’s super storm shine on

Finally, Elizabeth Zubritsky of Goddard Spaceflight Center has written a piece focussing on my colleague Brigette Hesman’s discovery of the gas ethylene within this hot stratospheric vortex:

NASA Spacecraft Sees Huge Burp at Saturn After Large Storm

Copyright: ESA/C. Carreau, full video can be obtained here.

Saturn's giant storm reveals the planet's churning atmosphere

Claudia Mignone, ESA

A recent study of the giant storm whirling on Saturn for the past two years, which became known as the "Great Springtime Storm", has given planetary scientists new clues about the planet's weather. Using a combination of data from the Cassini orbiter and ground-based telescopes, the scientists traced the storm's development from deep within the churning clouds in Saturn's lower atmosphere to altitudes hundreds of kilometres above the cloud decks, in the planet's stratosphere. There, two large pockets of warm air formed and later merged into one gigantic hot vortex that has been travelling around Saturn's northern hemisphere since mid-2011. The study of this storm and its associated vortex, which occurred unusually early in Saturn's 30-year-long weather cycle, suggests that waves play an important role in the energy transfer across the planet's atmosphere.

Storms are large disturbances in a planetary atmosphere. A common phenomenon on Earth, storms are not unique to our planet's weather and may arise on any planet that is surrounded by a thick atmosphere. Astronomical records report similar events on several planets in the Solar System, and recent data hint at possible storms on exoplanets.

A new study, based on data from the NASA/ESA/ASI Cassini-Huygens mission and ground-based telescopes, has looked into one of the largest storms recorded in the Solar System, which started whirling over Saturn's mid-northern latitudes about two years ago. The storm originated in the planet's lower atmosphere, where it was first seen in December 2010, and later grew to encircle the entire planet. The disturbance also propagated to higher atmospheric layers, where its aftermath can still be detected. It is known as the 'Great Springtime Storm' because it took place during the spring season in the planet's northern hemisphere, which started in August 2009 and lasts about seven years.

"Giant storms on Saturn occur regularly and have been observed for over a century, but this is the first time we could follow the temporal evolution of such an event in great detail," notes Leigh Fletcher from the University of Oxford, UK. Fletcher has led an extensive study of the Great Springtime Storm using data gathered in the infrared portion of the electromagnetic spectrum by the Cassini spacecraft, which has been orbiting Saturn since 2004, as well as ESO's Very Large Telescope and NASA's Infrared Telescope Facility.

"The storm was first detected in the planet's lower atmosphere – the troposphere – via optical and radio observations. Then we looked for its signature at mid-infrared wavelengths," explains Fletcher.

"When we look at Saturn's atmosphere in optical wavelengths, we see the sunlight that is reflected by a haze layer located deep down in the troposphere. In the mid-infrared, instead, we directly measure the temperature of the atmosphere for many kilometres above the clouds. This allows us to peer through the three-dimensional structure of the atmosphere," he adds.

Observing at these longer wavelengths provided a drastically different view, and allowed Fletcher and his collaborators to probe how the storm had infiltrated the upper part of the atmosphere – the stratosphere upwards from the troposphere. The presence of Cassini in the saturnian system and its ability to perform mid-infrared observations has allowed the astronomers to monitor the evolution of this unique meteorological event in unprecedented detail.

Mid-infrared images from January 2011 showed that two large pockets of warm air had formed over the storm, in the stratosphere. These warm air masses, also referred to as 'beacons', were both moving westwards, although with different speeds, and remained clearly separated for a few months. Between April and June 2011, the two beacons merged and gave rise to a giant vortex of clockwise-swirling air – an anti-cyclone – with temperatures up to 221 K, hotter than the surrounding air by 70-80 K.

The huge anti-cyclone in Saturn's stratosphere had fully detached from the tropospheric disturbance that caused it in the first place. At its biggest, in late June 2011, the vortex covered about 62 000 km – almost one quarter of the planet's circumference at the mid-northern latitudes affected by the storm. At the same time, the storm in the troposphere, only visible at optical wavelengths, had almost ceased.

"We kept monitoring Saturn during the storm with the help of many small, ground-based telescopes operated by professional and amateur astronomers alike, and found no sign of the giant vortex in the optical data. Although the tropospheric storm was the underlying cause of this enormous vortex, the vortex subsequently evolved independently of events happening deeper down, and was still present long after the tropospheric storm was over," he adds.

Since July 2011, the giant hot vortex has been shrinking and cooling at a very slow pace. It is still present in Saturn's stratosphere, where it has shrunk to less than half of its greatest extent, and is expected to disappear completely in a couple of years.

The data analysed by Fletcher and his collaborators showed how the temperature, wind velocity and chemical composition varied within and around the giant vortex. This allowed them to unveil how the storm had evolved over several months, and to investigate the energy transfer mechanisms at play among the various layers of Saturn's atmosphere.

"We suspected that the weather in the lower atmosphere has an impact on what happens at much higher layers, hundreds of kilometres upwards, just as happens in Earth's atmosphere. Now we have evidence for this on Saturn," says Fletcher.

In Earth's atmosphere, storm-generated waves are known to transport air and energy across the atmosphere, including upwards to the stratosphere. It is possible that a similar mechanism has taken place on Saturn, too: wave-like perturbations, induced by the tropospheric storm, might have made their way upwards to the stratosphere, where they released their energy and caused the formation of the two beacons.

"What is unusual in this particular case is that the two beacons interacted with one another up in the stratosphere, giving rise to the giant vortex. How exactly this happened remains an open question that needs to be tackled via numerical simulations," comments Fletcher.

The timing of the storm is also quite puzzling. Since 1876, large disturbances have been observed on Saturn with striking regularity: once every 'saturnian' year, which lasts about 30 years, and always during the northern hemisphere's summer season. The last such storm on record dates back to 1990, and the next one was expected in 2020.

"The Great Springtime Storm is definitely ahead of schedule with respect to Saturn's standard storm cycle. It is still unclear whether this is an isolated event or a signal that the storm season on the planet started earlier than expected," comments Nicolas Altobelli, Cassini-Huygens Project Scientist at ESA.

"Cassini will keep monitoring Saturn's atmosphere from its vantage point. The mission will be operating until the northern summer solstice, which will take place in May 2017. The storm season on Saturn's northern hemisphere may not be over yet, and in this case we might be able to see other spectacular events in the next few years," Altobelli adds.

"If storms are detected on Saturn in the upcoming future, it will be important to verify whether these will also produce dramatic aftereffects such as the stratospheric vortex from 2011," Fletcher concludes.

Royal Society Fellowship

I won’t lie - the past twelve months have been stressful.  Not only have we had our noses to the grindstone preparing proposals for ESA’s next mission to Jupiter (JUICE), but we’ve also had a flood of data from Cassini and ground-based facilities concerning variability and seasonal processes on all of the giant planets in our solar system.  This was compounded by having to write multiple proposals to funding agencies to try to make sure that my job here in Oxford was secure for the next few years, as my Glasstone Science Fellowship (see my blog entry here) is now coming to an end after three years.

Proposal writing involves reading and re-reading the same motivational/technical text over and over again until you can see the words in your sleep.  Putting together a case for support that not only convinces yourself, but also your reviewers, that this science is timely and worth spending a lifetime on, is no small task. Then comes the lightning-fast interview, with suit and tie facing a large panel of experts looking to see what you’re made of; what your motivations are; and whether you have a future in this field.  So after all this, it’s all-the-sweeter to be finally featured on someone’s list of awarded fellowships, and possibly the most prestigious of them all:  The Royal Society

Royal Society announces prestigious University Research Fellowships for 2012

http://royalsociety.org/news/2012/new-URFs-2012/

I am one of 36 new research fellows appointed by the society this year, in fields as diverse as particle physics, cellular biology, ecology and quantum chemistry.  My own research area concerns (yep, you guessed it) the exploration of giant planets, both in our solar system and beyond, specifically looking at seasonal and other time-varying atmospheric phenomena, as well as compositional constraints on the origins and evolution of these planets.  I’m also delighted to say that I’ll remain as a Fellow of University College, Oxford, who were kind enough to offer me a Junior Research Fellowship when I received by Glasstone fellowship in 2009.  

So after months of proposals and interviews, I’m happy to say that you’ll all have to put up with me for a little while longer, as I get to continue studying the solar system from the safe environs of the dreaming spires.